Tomato mutants reveal root and shoot strigolactones involvement in branching and broomrape resistance

Abstract

Abstract The phytohormones strigolactones (SLs) control root and shoot branching and are exuded from roots into the rhizosphere to stimulate interaction with mycorrhizal fungi. The exuded SLs serve as signaling molecules for the germination of parasitic plants. The broomrape Phelipanche aegyptiaca is a widespread noxious weed in several crop plants, including tomato (Solanum lycopersicum). 'In silico' screening of tomato (M82) mutants identified three lines that showed significantly increased branching. Two mutants, SHOOT BRANCHING 1 (sb1) and SHOOT BRANCHING 2 (sb2), lack SLs due to loss of function mutations in the genes for the carotenoid cleavage dioxygenase enzymes CCD7, and CCD8, respectively. Beyond the increased branching, these mutants were completely resistant to infection by P. aegyptiaca. The third branching mutant, SHOOT BRANCHING 3 (sb3), carried a point mutation in the SLs receptor DWARF14 and was found to be highly susceptible to P. aegyptiaca. SL concentration in roots of the sb3 was two-fold higher than in the wild type due to the upregulation of transcription of SL biosynthesis genes. This phenomenon suggests that the steady-state level of SLs is regulated by a feedback mechanism that involves the SL signaling pathway. Grafting experiments showed that sb1 and sb2 rootstocks protected wild-type tomato scions from P. aegyptiaca infection without a significant yield loss, offering a solution to the broomrape crisis. These results also demonstrate that strigolactones synthesized in the shoots are involved in the control of shoot branching.